Currently, data traffic are boosting rapidly while there exist clear bandwidth limitations in low frequency bands. The wireless industry is seeking to use higher frequency bandwidth in order to reach even higher data rates.
Some foreseen future use cases require data rates in the order of 10 Gbps. Providing such large data rates in an energy-efficient manner requires bandwidth in the order of many 100 MHz or even a few GHz, preferably contiguous bandwidth to ease implementation. Such large spectrum holdings are realistically only available in the millimeter-wave bands. Densification together with operation at very wide bandwidths in the millimeter-wave bands constitutes the concept Ultra-Dense Networks, UDN. In addition, a UDN should provide                As much capacity abundance in wireless networks as is available on fibre access;        Operator managed1 network access of areas not adequately served by cellular, such as densely populated public spaces, enterprises and indoor spaces; 1 In this context an operator can be a traditional operator but also a non-traditional one, e.g. a building owner which operates a UDN.        Enhanced network energy efficiency in order to retain a low energy footprint also in the future despite the expected massive traffic increase;        Coordination within and with overlaid networks (if present) for ease of access, energy efficiency, and mobility.        
The main motivation for UDN is energy efficient provision of substantially increased data rates and capacity compared to today's mobile broadband networks. UDN deployment therefore mostly makes sense in areas with high demands on data rate and/or capacity.
Examples of such areas are private property or semi-public spaces such as corporate buildings, campus, hotels, libraries, public buildings like arenas, shops, airports, train stations, train cars, outdoor environments, e.g. parks and city centres; as well as home and small offices.
Given the dense deployment and large number of UDN access nodes, simple and unplanned network deployment is very important. Even integration of user-deployed UDN access nodes should be supported. One interesting aspect of UDN deployment could be backhaul provision to a variety of access technologies, including UDN, LTE, Wi-Fi, Machine-Type Communications, MTC, standards, etc.
A typical deployment for Ultra Dense Networks, UDNs, is highly populated areas. Example deployments are hot spots, office building, or downtown areas in cities, where demands for high data rate service are present. Hence, it is necessary for UDN to utilise a higher carrier frequency and a wider bandwidth in order to reach a high data rate.
Due to unpredictable placement, antenna tilting angles, and adaptive beam-forming of each access point, AP, nodes, flash-light like interference could be a top reason to prevent a stable and high performance of UDN unless media access control, MAC, provides a flexible and effective mechanism.
It is expected that radio channel of UDN usually has a large bandwidth, e.g., varying for several tens, hundreds of MHz to larger than multiple GHz. The interference management across a vast bandwidth, either in continuous or disjoint bands, could be potentially complicated if Medium Access Control, MAC, does not provide a simple but effective solution.
The market trend and features of UDN differ from those of conventional system in that the owner of UDN might not be an operator company with a great expertise in cell planning and dominates the radio service in an area. The owners of UDN could be of distinct businesses, for instance, a real estate owner might own a UDN network, and install or improve or expand the UDN in their premises. Though they might get consulting from radio industry, however, a large portion of activities and decisions on maintenance and installation could be at their discretion.
Then, their UDNs' coverage could be partially or totally overlapping with each other. This put UDNs into a radio environment of full of possibility of interferences. As the involving node number is large, the interference manage method of MAC at UDN has to be simple and efficient.
Currently, Institute of Electrical and Electronics Engineers, IEEE, specification IEEE-802.11 system mainly runs in the unlicensed frequency band. In recent standard of IEEE 802.11, nodes, either terminals or APs, are competing resource via a mechanism of “listen before talk”. That is, a node can transmit signal only when the detected transmission, TX, is lower than a certain predefined threshold in order to avoid colliding of signals.
This is implemented using the so called Distributed Coordination Function, DCF, wherein each node waits a random back-off before accessing the channel, allowing other nodes to get an on the long term fair chance to access the channel. If another second node has a back-off time that becomes zero before the first node, the first node notices that the second node has started transmitting, by sensing the channel, e.g. listening to the channel, it postpones its transmission. At the next transmission possibility the node continues to count down the back-off time until it is zero. When the back-off time timer expires, i.e. the back-off timer becomes zero, the node performs its transmission.
Upon a collision, the nodes increase, up to a certain limit, their sensing, i.e. back-off, time in order to avoid further colliding to a large extent. In addition, each transmission contains a time indication Network Allocation Vector, NAV, which indicates the channel occupation of the transaction. This mechanism is also referred to as virtual carrier sense. A neighbouring node which detects transmissions and decodes the NAV should start a timer with the period indicated by NAV and wait until the timer expires. When the timer expires, the node starts another sensing on the availability of channel.
Both these mechanisms can be complemented by Request To Send and Clear To Send signalling to further reduce the possibility of a hidden node where the transmitter and this node could not sense the activity of each other but this node interferes with the receiver. Also Request to Send and Clear to Send signalling include NAV. A node that intents to transmit data sends out an Request to Send and only if it also receives a Clear to Send, it will start with the actual data transmission. Due to the NAV included in Request to Send and Clear to Send, neighbouring nodes overhearing Request to Send and/or Clear to Send know that a transmission will start and defer their own channel access by the time indicated by NAV.
By such a method, each node has a chance to get the radio resource. Such a scheme works well when the traffic is rather low. In case of high traffic load, resource efficiency becomes rather low due to an increased number of collisions, due to an increased probability of collisions. In brief the 802.11 DCF does not scale too well with increasing number of contending nodes and offered traffic.